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An 18-year-old man was apprehended at the Dover ferry terminal, a main link to Europe, followed by a 21-year-old man in Hounslow in west London.

"This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday," said Dean Haydon, the Metropolitan Police's head of counter terrorism command.

"Detectives are carrying out extensive inquiries to determine the full facts behind the attack," he added.

Police are continuing to search a commercial property in Hounslow and two other addresses in Surrey, south-east England.